1. Field of the Invention
The present invention is generally directed to instruments used for spectroscopic analysis and more particularly to a multipass cell for gas analysis using a coherent optical source.
2. Description of the Related Art
Multipass cells are commonly used in the spectroscopic analysis of gases. The basic idea involves folding the path followed by the optical radiation so that it crosses the same volume of gas multiple times. This maximizes the optical pathlength while minimizing cell volume and physical size. Until recently most spectroscopic gas analysis was performed with incoherent radiation such as the modulated infrared radiation present in the sample region of an FTIR spectrometer. The cells used have typically been of the White cell type (see FIG. 1).
When using a White cell, the incoherent radiation is focused on the input aperture of the cell. (In FIG. 1, this is shown as a small reflector). After passing through this, it expands to a relatively large diameter where it is intercepted by a correspondingly large, curved reflector. This re-condenses the radiation to form an image roughly in the same plane as the input aperture. By using two large condensing curved mirrors and a third curved mirror between the input and output apertures, as illustrated, it is possible to arrange for multiple passes. In this design, the radiation is successively imaged at points along one or two lines on the mirror between the apertures. After a predetermined number of passes, the radiation leaves the cell through a second aperture.
An object of the invention, therefore, is to provide a multipass gas cell for use with a coherent (tunable laser) spectrometer that can be taken apart for cleaning and then reassembled and easily realigned.